Solid-state imaging devices are generally called image sensors and the like, and are largely categorized into CCD sensors or MOS sensors. For these solid-state imaging devices having photodiodes formed in silicon substrates, there are three defects especially when miniaturizing pixels.
(Material) The first defect is decline in sensitivity due to the performance limit of photodiodes affected by the material properties of silicon. For example, when 2 μm of green light having a wavelength of 550 nm is incident on silicon, only about 92% is absorbed. That is, even if a photodiode of 2 μm in depth is formed, characteristics of quantum efficiency of 92% or over cannot be obtained. To solve this problem, the depth of a photodiode may be increased. However, since pixels are minute, the aspect ratio of the depth direction and the lateral direction increases. This means that manufacturing is difficult. Therefore, it is difficult to manufacture a high-sensitive solid-state imaging device.
(Aperture ratio) The second defect is that since a photodiode and a transistor within a pixel are formed in the same plane, a large area cannot be secured for a photodiode. Specifically, the area of the photodiode is an area obtained by subtracting at least the area of the transistor from the area of the pixel. Since light is concentrated on photodiodes having the limited area, a microlens is usually provided for each pixel. However, it is impossible to concentrate on the photodiodes all of the light beams incident on a solid-state imaging device (usually, efficiency is around 50 to 70%). For a minuter pixel, the ratio of the area of a photodiode to the total thickness of a line and an interlayer on the pixel becomes smaller. This further deteriorates the efficiency. That is, in conventional solid-state imaging devices having photodiodes formed in silicon substrates, light which irradiated areas other than a photodiode, such as an amplifier transistor and an element isolation area formed on the substrate is not photoelectrically converted. This means a loss. Even with a means of arranging a microlens, light cannot be concentrated only on photodiodes. Therefore, this efficiency decline is inevitable.
The efficiency can be improved by irradiating the back on the line side with light i.e., by employing a backside illumination sensor. However, an efficiency of 100% is impossible. This is because photoelectric conversion is performed on an entire surface in backside illumination sensors or layered sensors, and thus there is no loss as caused in the conventional solid-state imaging devices. If loss caused by reflection or absorption from a light source to a photoelectric conversion film, which is also seen in the conventional solid-state imaging devices, can be ignored and the internal quantum efficiency of the photoelectric conversion film is 100%, then an efficiency of 100% may be possible. Although there are, in fact, no materials having an internal quantum efficiency of 100%, many materials are known as materials superior to Si which is a general substrate material for solid-state imaging devices. Solid-state imaging devices more efficient than conventional ones can be manufactured by combining such a material and, for example, a Si substrate most easily manufactured as the material of a control circuit. However, an internal quantum efficiency of 100% is impossible. Therefore, this also leads to decline in sensitivity.
(Decline in FD capacitance due to miniaturization results in limitation of the storage capacitance in a photodiode) The third defect is that a large storage capacitance cannot be secured for a photodiode. Normally, an electric charge is transferred from a photodiode in a pixel to a floating diffusion via a transfer gate. The gate of an amplifier transistor is connected to this floating diffusion, and a voltage according to the electric charge is outputted. Here, all the electric charges (electrons) generated in the photodiode need to be transferred to the floating diffusion, i.e., complete transfer is needed. To prevent a transfer omission, it is not possible to increase the ratio of the storage capacitance in the photodiode to the capacitance of the floating diffusion. Therefore, the saturation number of electrons in a pixel (maximum number of electrons detectable in a pixel) decreases. Thus, the dynamic range of the solid-state imaging device declines. This is more significant in minute pixels in which each capacitance is reduced by scaling.
An example of a solid-state imaging device which can overcome these problems is layered sensors disclosed in Patent Literatures 1 and 2, for example. As an example of this, FIGS. 7 and 8 illustrate a layered sensor disclosed in Patent Literature 1. FIG. 7 is a cross-sectional view corresponding to the frame of a broken line in FIG. 8.
As shown in FIGS. 7 and 8, in the layered sensor, a photoelectric conversion unit (18 in FIG. 7) is formed above a transistor. This is based on an assumption that the upper surface of the photoelectric conversion unit is a surface on which light is incident. Therefore, the second restriction described above is eliminated, and photoelectric conversion can be performed on the enter surface. Furthermore, since the photoelectric conversion unit can be made of a material superior to silicon in photoelectric conversion properties, the first restriction is also eliminated. Furthermore, there is a metal (pixel electrode) for connecting a photoelectric conversion film and a control circuit in the layered sensor, and the electrons in the metal are not depleted. Therefore, the complete transfer described in the third defect is not possible. However, as the reference potential of the metal can be arbitrarily set to a high or low potential, it is also possible to design larger storage capacitance.